At present, silicon-iron is a standard for use in most induction devices since it combines a high saturation flux density with relatively high electrical resistivity, magnetic permeability and low hysteresis loss. Silicon-iron is also attractive because it is relatively inexpensive. To date, silicon-iron is the primary material in use in transformer core laminations in the primary power net in the United States.
Even though it does possess favorable characteristics, transformers in the primary power net in the United States which incorporate this material account for about a 3.1% loss of power transmitted, with approximately 1.5% lost in the core and 1.5% lost in the transformer winding (in transformer design, system loss is generally minimized if the design is arranged so that core and resistive losses are equal). The losses in the core have an economic value of nearly half a billion dollars annually.
There have been developed a number of glassy alloy materials which have unusual ferromagnetic properties. The materials are amorphous, i.e. they do not possess a crystal structure. The lack of crystalline structure in amorphous (glassy) metals is associated with a high degree of magnetic softness, a property which is desirable in any material intended to be used as an inductor. Glassy metals can generally be fabricated by rapidly quenching (also called splat cooling) an alloy which is usually about 20% of a metalloid such as carbon, silicon, phosphorus or boron and about 80% of a metal such as iron. The cooling rate is designed to be so rapid that a crystal structure does not have time to form.
In addition to rapid quenching, amorphous materials can also be realized by vacuum deposition, sputtering, plasma spraying, electrodeposition, and electroless deposition. Whatever the means of preparation, however, amorphous materials are generally realized by alloying the metal and metalloid in proportions such that a eutectic would be found in ordinary metallurgy.
The realization that certain glassy materials have soft magnetic properties suggested that they could offer a potentially large reduction in core losses in power distribution transformers with attendant savings of up to several hundred million dollars, as discussed above. The potential savings have led to interest in such systems and have led to considerable inquiry in both material and device fabrication.
The development of low stress amorphous metal electroforms having reasonable thicknesses along with low edge-to-edge and end-to-end thickness variation and minimal surface roughness would therefore be useful to the electrical industry. However, rapidly quenched materials are limited with regard to the thickness of material that can be realized. The inherent nature of rapid quenching involves heat transfer and, to date, with the exception of some rather cumbersome shock techniques, the cooling requirement restricts the maximum realizable thickness of quenched iron-phosphorus to approximately one-thousandth of an inch. Rapidly quenched materials also sometimes exhibit considerable edge-to-edge and end-to-end thickness variations along with surface roughness.
It is believed that thick iron-phosphorus electroforms would be particularly useful in fabricating, e.g. transformers using conventional laminating methodology used to make silicon-iron cores. Rapidly quenched iron-phosphorus is too thin to be laminated into a core material, and is instead formed by a fairly complicated process into a core torus through which metal conductors (e.g. copper) are wound by a process called re-entrant winding. Re-entrant winding is much more complicated and harder to implement than conventional core lamination processes. And, using the thin layers produced by rapid quenching, a great number of thin, rapidly quenched layers would be required to obtain a core of normal size. Thus, because rapid quenching cannot achieve thick layers of iron-phosphorus (i.e. iron-phosphorus electroforms), conventional core fabricating methodology is not used with rapidly quenched materials.
To date, the art has not developed, to the inventor's knowledge, a practical electrochemical method which allows plating iron-phosphorus layers. In particular, electroforming would seem to be a desirable means of fabricating iron-phosphorus layers or laminations inasmuch as, by controlling electrodeposition parameters (such as current density), bath conditions (such as temperature, pH etc.) and material balance, careful control of plating and electroforming operations can be maintained. However, very few researchers have developed any method at all for plating amorphous iron-phosphorus, let alone a practical method which allows plating low stress iron-phosphorus having good (i.e. low) thickness variations and little if any surface roughness.
For example U.S. Pat. No. 4,101,389 to Uedaira discloses the production of amorphous iron-phosphorus from an iron (0.3 to 1.7 molar divalent iron) and hypophosphite (0.07-0.42 molar hypophosphite) bath using current densities between 3 and 20 amps/square decimeter (between 30 and 200 milliamps/cm.sup.2) a pH range of 1.0-2.2, and a temperature range of 30.degree. C. At the low pH values generally disclosed therein, however, a disadvantage is that redissolution of the iron occurs. Further, within the temperature range cited it is difficult to obtain deposits free of oxide inclusions. Such inclusions cause the plating to crumble readily and thereby render it unsuitable for use.
U.S. Pat. No. 3,086,927 to Chessin et al discloses the addition of minor amounts of hypophosphite to an iron plating bath to harden the iron. This patent cites using between 0.06 and 6 grams/liter of hypophosphite at a temperature between 100.degree.-170.degree. F. over a current density range of 20 to 100 amps/ft.sup.2. The disadvantage with this method is that the hydrogen overvoltage for iron declines sharply as hypophosphite is added to the bath. Both plating efficiency and throw suffer and the quality of the plating declines markedly as hypophosphite is increased in the bath.
A bath which could be used to achieve amorphous iron-phosphorus electroplating practically, i.e. with low stress and over a wide range of current density, would thus provide a means for achieving useful amorphous iron-phosphorus platings and electroforms which, due to the fact that they were low stress, could be plated more thickly than permitted by the rapid quenching prior art. Such a bath would also permit fabricating useful inductive devices by presently employed methods. The present invention provides a method which, it is believed, allows achieving low stress amorphous iron-phosphorus electroplatings and electroforms over a wide range of current density values.